GLASS. 



the fifei wliich at firft. was only of charcoal, is to he of 

 dry wood : fo that the flame covers the whole pan, and 

 evjn ifTiies out at the chimnics. 



Durin<r the lad hours, they make affays from time to 

 time, by taking out pieces laid for that purnofe, through 

 the little aperture of the furnace, and pan, to ky whe- 

 ther the yellow be p"rfeft, and the other colours in good 

 order. When the annealing is thought fufficicnt, thry 

 proceed with great haile to cxtinguifli the fire, wliich 

 ctherwifc would foon burn the colours, and break the 

 glaffes. See Paintikg. 



Glass, pahiting on, f>y means vf prints. See V> \r.Vi-{\vnl- 

 iitg. 



Gr.A??, poUfhln^ of- See PoilstriKC and GniNDiXG. 



GL.Kf^-^-porCiLiin, tlie name given by many to a modern 

 invention of imitating the china-ware with glafs. The nie- 

 thod given by M. Reaumur, who wa"; the tirll that carried 

 the attempt to any degree of perfeftion, if delivered by that 

 gentleman in the Memoirs of the Academy of Seienees of 

 Paris, to this eKeti. This change of glafs was tiril taken 

 notice of by Neumann, who, in dillillmg milk in a glafs 

 retort, obferved, that the bottom of the veflel acquired the 

 appearance of porcelain, wliich he attributes to the line 

 white earthy matter of the milk forced into the glals by heat. 

 Neum. Chem. Works by Dr. Lewis, p. 571. 



The mixing of glafs reduced to powder, with other Icfs 

 eafdy vitrifiable fubftances for forming a pafle, to be after- 

 wards made into a fort of a porcelain, has been a contrivance 

 long praftifed, but it is very troublefome, and the refult 

 fubjetf to many faults ; but this new ware is made of glafs 

 alone, and that with much lefs trouble, and without tlie 

 reducing it to powder. By this art, veffels of glafs are 

 changed into veltels of a fort of porcelain, without altering 

 their'form, and the nieanelt glafs miule only of fand, lime, 

 and fahne adies, ferves as well as the bell for that purpofe : 

 our common coarfe green quart-bottles, or the great bell 

 glalTes with wh.ieh gardeners cover their melons, S:c. being 

 by this means changeable into a beantifid white fort of por- 

 celain ware ; and this is to be done in fo cafy a manner, and 

 with fo fmall expence, that it requires no more trouble or 

 charo-e, than that of baking a common veffel of our coarfe 

 earthen-ware ; and for this reafon the veffels ot this fort of 

 ware may be afforded extremely cheap. 



It is very certain, that all p.ircelain ware is a fubrtance 

 in the ftate of fe-rii-vitrification : and in order to bring glafs, 

 which is a wholly vitrified fubffancc, into the condition of 

 porcelain, there requires no mure than to reduce it to a leis 

 perfeftly vitrified ftate. 



The queftion which would naturally be ftarted on this oc- 

 cafion, is, wjhether it be poflible to reduce glafs to a lefs 

 vitrified ffate, it having already undergone what is ellcemed 

 the laff change by fire. But when we confider, that the 

 mafs of antimony, the vitritieations wf many of the metals, 

 as the glafs of lead, and the counterfeit gems coloured by 

 (he metals, are more or. lefs eafily reduced again by che- 

 ir.iltry to metals, &c. tlie r^-dueing of fand, flints, &c. after 

 ti-iey are vitrified, at leall a little way back tovard their 

 native or prilline date, niay appear not wholly i'.npr;idieahle, 

 and the attempts which M. Reaumur made on this occafion, 

 were what gave him the firll hints of the glafs-porcelain ; 

 called from his'name " Reaumur's porcelain." 



The method of making it is this. The glafs veffels to be 

 converted into procelain are to be put into a large eartheu 

 veffel, fueh as the common line earthen didies are baked in, 

 «;r into fufhciently large crucibles ; tlie veffels are to be filled 

 with a mixture of fine white fand, and of fine gypfum or 

 tU.icr-ftone burnt into what is called p laftcr of Paris, ami 



all the interftices are to be filled tip with the fame powdcr» 

 fo that the glafs veffels may no where touch either one an- 

 other, or the hdcs of the velfel they are baked in. The veffel 

 is to be then covered dowji and luted, and the fire does the 

 red of the work ; for this is only to be put into a common 

 potter's furnace, and wlien it has flood there tlip ufual time 

 for the baking of the other veffel.i, it is to be taken out, and 

 the whole contents will be found no longer glafs, but con- 

 verted into a white opaque fubllance, which is a very elegant 

 porcelain, and ha": ahnoll the properties of that of China. 

 M''moirs Acad. Sciences Par. 1 739. 



The powder which has ferved once, will do again as well 

 as frelh, and that for a great many times : nay, it feems, 

 ever fo often. The caule of this transformation, fays Mac- 

 quer, is pVobably that the vitriolic acid ot the gypfum quits 

 its bafis of calcareous eartli, and unites with the alkalme fait 

 and faline earth of the glafs, with which it forms a kind of 

 fait or felenites, differing from the calcareous felenites, by 

 the interpofition of which matter the glafs acquires the 

 qualities of porcelain. Dr. Lewis, from a variety of ex- 

 periments on the nature and qualities, and method of pro- 

 ducing this porcelain, has deduced the following conchi- 

 fiuns : I . Green glafs cemented with white fand received no 

 change in a heat below ignition : in a low red-heat, the 

 change proceeded very llowly ; but in a llrong red-heat, ap- 

 proaching to whitenefs, the thickell jjieces of glafs bottles 

 were thoroughly changed in three hours. 2. The glafs 

 fullained the tuUowing progreiTiou of changes. Its furface 

 firft became blue ; its tranfparence was diminiflied, and 

 v>hen held between the light and the eye, it appeared of a 

 yellowilh hue : afterwards it was changed a little way on 

 both fides into a white fubllance, externally Hill blueifli : and 

 as this change adviinccd Hill farther and fartlier within the 

 glafs, the colour of the vitreous part in the middle ap- 

 proached nearer to yellow : the white coat was ot a fine 

 fibrous texture, and the fibres were difpofed nearly parallel 

 to one another, a'ld tranfverfe to the thicknefs of the piece : 

 by degrees the glafs became throughout white and iibrous, 

 the external blneilhncfs at the fame time going off, and being 

 fucceeded by a dull whitidi or dun colour : by a ftdl l<;nger 

 continuance in the fire, the fibres were changed gradually 

 from the external to the internal part, and converted into 

 grains ; and the texture then was not unlike that of common 

 porcelain. The grains, at firll fine and fomcwhat gloffy, 

 appeared afterwards larger and duller, and at length the 

 fubilance of the glafs became porous and friable, like a 

 mafs of white fand (lightly cohering. 3. Concerning the 

 qualities of the converted glafs Dr. Lewis obferves, that the 

 whitenefs of tlie internal part was noj inferior to that of 

 porcelain, but that its furface was the Icall beautifij ; that 

 the thick pieces were quite opaque, and that feveral thin 

 pieces were femi-tranfjiarent : that while it remained in a 

 fibrous (late, its liardnefs became greater than tiiat of glafs, 

 or of the common kinds of porcelain ; it was capable of 

 fiiflaining fudden changes of heat and cold better than any 

 porcelain ; and in a moderate white heat, it was fuiible into 

 a fubllance not fibrous, but vitreous and fmooth, like wliite 

 enamel : that when its texture had been eoarfely granulated, 

 it was now much fofter and unfulible ; and, lallly, that 

 when fome eoarfely granula'ed unful.ble pieces which, with 

 the continuance ol a moderate heat, would have become 

 porous and friable, wi're iuddeidy expoivd to an intcnfe tire, 

 they were rendered remarkably more compacf than before ; 

 the folidity of fome of them being fuperior to that of any 

 other ware. 4: No di.ffei'ences appeared in the intirnal 

 colour, hardncfs, texture, or the regular fucccffum of 

 changer-, 'from the ule of Jiffeieiit cementing fubilances ;' 



thou'jh 



